Gonadal hormones can produce striking behavioral and neural plasticity in adult organisms. For example, systemic administration of testosterone to adult female canaries induce the development of male-typical vocalizations and results in a striking increase in the size of brain nuclei that are known to be involved with vocal control. We have shown that testosterone (T) treatment increases the volume of the vocal-control nucleus HVc (caudal nucleus of the ventral hyperstriatum) in hearing adult females only; T-induced growth of HVc is greatly attenuated in birds that are deprived of auditory stimulation. Thus, testosterone treatment alone is not a sufficient stimulus for neural growth in HVc. This result suggests that T does not stimulate growth solely via a direct action on hormone receptors in HVc, but rather that testosterone and sensory stimulation can act synergistically to produce structural plasticity in the adult brain. We propose several experiments that will begin to elucidate the mechanisms whereby gonadal hormones and auditory stimulation interact to engender neural changes that underly the capacity for vocal learning. We will: (1) determine whether differences in the volumes of song-control nuclei between normal, deafened, T-treated hearing, and T-treated deafened birds are due to differences in neuronal number or density; (2) examine the dendritic morphology of song-control neurons in these four treatment groups in order to determine what effect auditory and hormonal stimulation have on dendritic growth; (3) determine the time course of growth in HVc of hearing vs. deafened T-treated birds in order to assess whether auditory afferent input exerts an inductive or a sustaining effect on neural growth; (4) begin to define the functional characteristics of androgen target cells in HVc by determining their axonal connections; (5) directly measure differences in androgen accumulation in HVc among our four treatment groups to determine whether variations in auditory afferent input can modulate the ability of cells to accumulate hormone. These experiments will provide fundamental data relating to neural and hormonal mechanisms of vocal learning, as well as new insights into treatments for hearing-impaired persons.